The long term goal of this project is to contribute to an understanding neuronal coding and computation. Such information is particularly relevant to the design of neural prosthetic devices (e.g., artificial cochleae) and their interfaces to the nervous system. As a model, the circuitry by which the barn owl's auditory system calculates interaural level difference (ILD), i.e., the difference in the loudness of the sound in the ears, will be analyzed. The barn owl is a species of choice because the significance of ILD to its behavior is clearly understood: ILD signifies the vertical co-ordinate from which a sound emanates. Moreover, its auditory pathways have been traced and characterized. In this project, attention will be focused on a nucleus of the owl's brainstem, VLVp, that is the first site at which ILD is computed. Preliminary studies indicate that the computation requires an inhibition that originates in VLVp of the opposite side. Thus, the central task is to identify and characterize the inhibitory neurons that project to the opposite nucleus. This will be done by a combination of axonal tracing methods and by selectively anesthetizing a subdivision thought to contain the inhibitory neuron. The topography of the projection from this subdivision will be analyzed and compared with a gradient of inhibitory strength that is known to exist in VLVp. Finally, an attempt will be made to study the effects of altering the sensitivity of VLVp neurons to ILD on higher auditory centers. For the latter project, the neurotransmitter that mediates the inhibition will be identified. The study of VLVp's circuitry is essentially a study of a neural system that contrasts the strength of inputs (e.g., from the two ears), and the kind of morphology that underlies this computation. Thus, resulting data will have significance to the study of neural systems in general.